Polycrystalline diamond (PCD)

Polycrystalline diamond (PCD) is a kind of new superhard material which is studied and researched in the 1970s. It can be sintered at a high temperature of 1400℃and high pressure of 6GPa.

Polycrystalline diamond is not only a kind of new engineering material but also a kind of new functional material. It is not only a high-tech product but also a high-benefit product. With the development of modern industry and science and technology, polycrystalline diamond has been widely used in modern industry, national defense and high-tech fields because of it”s excellent mechanical, thermal, chemical, acoustic, optical and electrical properties.

The Polycrystalline diamond tool has become an indispensable tool in the modern cutting process, which is mainly shown in the following aspects:

1. High-speed cutting, high stability processing

2. Ultra-precision mirror processing

3. Dry cutting, clean machining

1 PCD performance Introduction

1.1 High hardness and wear resistance

The hardness of polycrystalline diamond is up to 10,000HV, which is the hardest material in artificial materials in the world. It is much harder than tungsten carbide and engineering ceramics. Also polycrystalline diamond has excellent wear resistance.

1.2 Low friction coefficient

The friction coefficient of polycrystalline diamond and some non-ferrous metals is lower than that of other materials, which is about 1/2 of tungsten carbide. Low friction coefficient not only makes deformation and reduces cutting force. Also does not produce chips during cutting, thus reducing the surface roughness.

1.3 High thermal conductivity

Polycrystalline diamond has a high thermal conductivity, better than silver and copper, and much higher than tungsten carbide. So, cutting heat dissipates easily and the cutting temperature is reduced effectively when cutting.

1.4 High machining accuracy

1.4.1 Because polycrystalline diamond tools have low thermal expansion coefficient and high modulus of elasticity, so they do not deform easily and keep the sharpness and high cutting accuracy when cutting. When PCD tool is used to process, it reduces

cutting force and cutting temperature, and improves tool durability and cutting rate, and get a good machined surface.

1.4.2 Machining problems: The high hardness and wear resistance of PCD materials make form and finish difficult, which seriously hinders its popularization and application.

Common machining methods: electric discharge machining (EDM), Laser machining, chemical machining, ultrasound machining

Ideal machining method: grinding or lapping

2. Introduction of PCD processing technology

2.1 Grinding

In view of high hardness and wear resistance of polycrystalline diamond, the grinding ways mainly are diamond grinding wheel grinding, electric discharge grinding and electrochemical grinding. Diamond wheel grinding is the simplest and most effective grinding method. In the actual machining process, apply to different grinding methods to combine machining.

2.1.1 Diamond Wheel Grinding

When diamond wheel grinding polycrystalline diamond, the direct contact pressure and grinding pressure between the grinding wheel and workpiece are high, so it requires the grinding system has sufficient rigidity and high precision. In recent years, many countries are researching and developing special grinding machines and grinding wheels for polycrystalline diamond (including polycrystalline cubic boron nitride PCBN). In the early stage of development resin bond diamond grinding wheels are used, but the grinding efficiency is low. In recent years, with the development of the technology of vitrified bond diamond grinding wheel, its comprehensive performance far exceeds resin bond grinding wheel. Therefore, many vitrified bonded diamond grinding wheels for PCD and PCBN have been applied, replacing most of the resin bond diamond grinding wheels.

2.1.2 Mirror grinding of polycrystalline diamond

Electrolytic in-process dressing (ELID) precision mirror grinding technology is a kind new ultra-precision machining technology developed by Japan Institute of Physics and Chemistry in the late 1980s. Its basic principle is that use the dynamic balance between the non-linear electrolytic dressing and the electrolytic inhibition of the oxide insulating layer on the surface of the metal bond superhard grinding wheel to dress the grinding wheel continuously. So the grinding wheel can obtain constant outburst. And it can be continuously processed in the optimum grinding state. It is suitable for the precision mirror of hard and brittle materials.

Metal bond diamond grinding wheel and ELID precision mirror grinding technology are used for polycrystalline diamond, and it can achieve good results. It is a new way of polycrystalline diamond precision machining.

2.1.3 Electrical discharge grinding(EDG)

In the process of EDM, the characteristics of the pulse are very important, especially in the processing of PCD materials. PCD has a certain conductivity, good thermal conductivity, and high melting point, so it is very difficult to process polycrystalline diamond by traditional EDM. The erosion mechanism of electric discharge grinding for PCD includes gasification of diamond, oxidation of diamond, graphite and amorphous carbon conversion of diamond, throwing force by electric discharge, cracks generated by thermal stress on diamond surface and fracture and breakage of diamond grain. The graphitization of diamond plays a key role in the process of electric discharge polishing diamond film. It not only conducts electricity and maintains the discharge channels. And it makes a diamond film can be gasified at the top of prominent peaks. With the graphitization-oxidation process of diamond going, the diamond can be eroded.

Electrical discharge grinding of polycrystalline diamond is an efficient and low-cost method, but it cannot effectively process large areas of polycrystalline diamond.

2.2 Lapping

There are three main lapping methods for polycrystalline diamond: one is precise grinding with diamond grinding wheel; one is grinding with a quenched high hardness steel plate (or agate plate) without any abrasive; the other is grinding with high speed rotating cast iron plate, supplemented by diamond powder. The lapping process can be used as a finish polishing process of polycrystalline diamond, and it is an important part of manufacturing polycrystalline diamond tools.

2.2.1 Lapping of Diamond Wheels

The lapping of PCD with a diamond grinding wheel is similar to that of grinding with a grinding wheel. But choose a different type of grinding wheel. The vitrified bond diamond grinding wheel is usually used for rough grinding. If the surface finish is required to be higher, fine resin bond grinding wheel can be used for grinding. When grinding polycrystalline diamond with diamond grinding wheels, because the process of diamond abrasive grain lapping the PCD materials is that two kinds of hardness and similar objects interact each other, and is essentially different from the traditional grinding process. Therefore, the PCD lapping mechanism and lapping process have their own features.

2.2.2 High-speed steel disc for polycrystalline diamond

The basic principle is friction occurs between PCD workpiece under certain pressure and high speed rotating high-speed steel disc, resulting in friction heat, which increases the temperature of grinding surface. The high temperature on the grinding surface is good for oxidation, graphitization, diffusion, bonding and thermal stress breaking of diamond grains on the grinding surface in PCD, so as to achieve the purpose of grinding.

According to the chemical properties of diamond, some metal elements can react with diamond and promote its disintegration. If the abrasive disc itself contains the above elements, it will promote the diffusion, bonding and thermal erosion of diamond on the abrasive surface. It helps to improve the efficiency of abrasive processing, but the temperature of the abrasive surface should not be too high, or else it will affect the surface quality of lapping.

2.2.3 Conventional abrasive grinding for polycrystalline diamond

Conventional abrasive grinding for polycrystalline diamond is one of the traditional processing methods. In the process of conventional abrasive grinding, the brittle fracture of abrasive grain is one of the main removal methods in the process of PCD grinding. Sharp abrasive grains scratch PCD at high speed and appropriate pressure, resulting on many damage scratches on the surface of the diamond grain. These scratches can be divided into plastic scratches and brittle scratches. This method is also an important mechanical removal for PCD materials.

2.3 Other Processing Methods

2.3.1 Electrical discharge machining (EDM)

Electrical discharge machining can cut materials by the pulse discharge in the local area between the electrode wire and the workpiece. It is a common method in PCD cutting. Wire cut electrical discharge machining PCD is mostly used to make tools, geological drill bits, and polycrystalline diamond dies. These workpiece abrasive are complex and various in shapes. The forming grinding by electrical discharge cutting machine is a good economic way.

2.3.2 Laser machining

The principle of laser machining PCD is that a laser beam with high energy density irradiates the PCD surface, and part of the light energy is absorbed by the surface and transformed into heat energy. The local temperature of the irradiated spots rises rapidly to 10000℃, making the PCD melt locally or even gasify and form sink. While, thermal diffusion begins, resulting in the materials abound the spots melt. With the continuous absorption of laser energy, vapor expands and pressure increases in the sink. The melt is ejected at high speed in the form of an explosion. The back-pressing force produced by the ejection forms a strong shock wave in the inner part of the workpiece. In this way, polycrystalline diamond erodes part of the material under the action of high temperature melting gasification and shock wave, forming laser etching sinks.

Laser machining is non-contact machining. It has excellent characteristics in PCD material processing. It can process high purity PCD material (such as polycrystalline diamond film). The processing deformation and thermal deformation is very small. It is fast and efficient for laser machining, and it is a more effective processing method. Especially in the processing of micro hole and non-conductive superhard materials, it has its superiority. However, this method also has its shortcomings. It is difficult to control the dimension accuracy and repetitive accuracy of the workpiece, and there is micro graphitization on the machined surface, which can be dressed. So, this method can only be used for rough and semi-finish processing.

2.3.3 Chemical machining

The chemical machining of PCD is based on the fact that diamond is soluble in molten potassium nitrate, sodium nitrate, and sodium phosphate. The surface of PCD can be changed from rough to smooth by dissolving PCD with molten nitrate. While, because of the oxidation reaction, a kind of oxide film can be formed on the surface of PCD, which can be removed by grinding and then continue to dissolve.

2.3.4 Ultrasound machining

Ultrasound machining of PCD is that apply diamond powder(1-50um) as free abrasive grains. The main factors affecting ultrasonic processing of PCD materials are vibration frequency, amplitude, diamond powder size, PCD material, unit pressure, processing area, etc. At present, PCD ultrasonic machining includes ultrasonic grinding and ultrasonic lapping. It is mostly used for PCD inner hole and PCD dies, sandblast die.

3.Introduction of applications

PCD tool has very high durability, which is tens of times longer than that of tungsten carbide tools, stable dimensional processing accuracy and good surface roughness of the workpiece. The main processing objects are non-ferrous metals, non-metallic materials, and woodworking materials containingAL2O3, alloys, ceramics, various fiber and particle reinforced composite materials. Ultra-precision processing can also be done for the above materials. Nowadays, the main are PCD woodworking saw blades and trimming knives, PCD inserts, PCD piston series cutters, PCD hub cutters, etc.

3.1 Automobile industry

PCD tools account for 60% of the applications in the automotive field. PCD tools are mainly used to process engine piston, pinhole, cylinder block, gearbox, carburetor and etc.. Because the silicon content of these parts is high (more than 10%) and most of them are produced in batches by pipeline mode and require high service life of the tools. And tungsten carbide tools cannot work. The durability of diamond tools is 10-50 times that of carbide tools. It can ensure the dimensional stability of the parts and improve the cutting speed, efficiency and surface quality of the workpiece

3.2 Wood processing industry

Recently, the wood-based panels widely used are obviously different from traditional wood. In a sense, synthetic panels are the concept of synthetic resin, but the process is different from traditional wood. In particular, the development of wood-based panels such as medium density fiberboard, plywood, particleboard, and composite flooring has accelerated the need for superhard cutting tools. Diamond cutting tools have gradually replaced the traditional woodworking tools in the market. The AL2O3 on the outermost layer of reinforced composite flooring greatly affects the wear of carbide tools.

3.3 Electronic industry

Usually, the cutting tool easily rolls up when cutting the edge of the printed board, which affects the installation and positioning of various electrical components, and the whole printed board is not beautiful. The carbide tool has a short service life, slow speed, low efficiency and poor accuracy in the processing of electrical printed boards. PCD multi-edged electric printing plate cutter does not wear easily, and the speed is up to 1500-2000M/S. Therefore, it has high efficiency and low cost and has been widely used.

3.4 Petroleum geological drilling industry

Polycrystalline diamond geological drill bits can achieve high footage(4-6 times higher than cone bits) and high drilling speed(2 times higher than core bits) at low bit pressure( about 40KN). It has higher safety than core bit, and can greatly improve drilling efficiency and reduce drilling costs. PCD drill bits have outstanding advantages in large soft to medium hardness formations, especially in high-cost offshore drilling, ultra-deep drilling, and slim hole drilling.

3.5 Wire Die

Wire drawing die is a very important die for wire drawing of various metal wire manufacturers (such as wire and cable factory, a steel wire factory, welding rod, and wire factory, etc.). Wire drawing dies are widely used in drawing bar, wire, pipe and other straight lines difficult-to-process materials. They are used in drawing of steel, copper, tungsten, etc.

When drawing copper wires with the same diameter, the service life of polycrystalline diamond die is 300-500 times longer than that of tungsten carbide die.

3.6 Glass cutting

PCD cutter wheel is mainly used for high precision cutting of liquid crystal glass in the electronics industry. It can also cut building glass and automobiles glass. Its mandrel and cutter wheel is PCD materials, so it features high accuracy, wear resistance, good consistency, and high service life. It is an ideal replacement of conventional glass knife and carbide glass cutter wheel.

3.7 Gemstone processing

Compared with the traditional electroplated diamond grinding wheel, the polycrystalline diamond tool has a long service life in the gemstone processing. In many cases, the working layer of electroplated diamond is single layer diamond, when grinding high hardness diamond, and it is quickly worn and cannot work. Polycrystalline diamond completely overcomes this weakness and greatly improves the working efficiency. Gemstone processing is a newly developed application for polycrystalline diamond cutting tools in recent years. It has a promising prospect and greatly broadens the market of polycrystalline diamond. The hardness of this new type of diamond is lower than that of drilling and other fields, and some special treatments have been made for gemstone processing.

4. Development requirements and trends of PCD

4.1 Size is bigger and bigger.

4.2 Grain refinement, quality optimization, and performance homogenization. Early polycrystalline diamond products are applied 50um diamond powder, but now develops 2um or even less than 0.5um, so that polycrystalline diamond tools, dies inaccuracy is no longer inferior to single crystal diamond.

4.3 The wear ratio of PCD is higher and higher. The wear resistance of polycrystalline diamond is an important index to measure its quality level. As a new type of superhard material product, its quality level has been continuously improved, and the wear ratio has become higher and higher after years of research and production.

4.4 Various shape and structure of PCD products.

In the past, polycrystalline diamond products were usually slice and cylindrical. With the development of large size and processing technology (such as EDM and laser cutting), triangle, herringbone, gable spherical surface, surface, and other special shapes increased. In order to meet the needs of special cutting tools, wrapped, sandwich and coil polycrystals appeared.